1. Field of the Invention
The present invention relates to exposure apparatuses, and more particularly though not exclusively, to an exposure apparatus that projects a pattern provided on a reticle (photomask) onto a substrate by exposure in lithography for manufacturing devices such as semiconductor devices and liquid crystal display devices.
2. Description of the Related Art
In recent lithographic processes for manufacturing semiconductor devices and so on, an exposure apparatus has been used to transfer a reticle pattern onto a wafer on which a resist is applied.
Further, the level of integration of integrated circuits has been enhanced, that is, circuit patterns have become finer, and the wavelength of exposure light used in the exposure apparatus has been shortened in order to achieve high resolution. Accordingly, a KrF excimer laser with a wavelength of 248 nm or an ArF excimer laser with a wavelength of 193 nm is currently being used as a light source in the exposure apparatus, instead of a mercury lamp that has been dominant until recently.
On the other hand, high definition can be achieved by increasing the numerical aperture (NA) of a projection optical system in the exposure apparatus. Accordingly, an immersion exposure apparatus has been discussed in which the space between the projection optical system and the wafer is filled with a liquid having a refractive index of more than 1 in order to increase the numerical aperture and to thereby increase the achievable density of circuit patterns.
With increases in the definition of the exposure apparatus, strict requirements have been imposed on aberration correction of the projection optical system installed in the exposure apparatus. For this reason, it can be necessary to measure and inspect the optical characteristics of the projection optical system.
For example, it can be necessary to measure a subtle change in the optical characteristics of the projection optical system caused during transportation of the exposure apparatus and to readjust the optical characteristics to obtain the best characteristics. The necessity of measuring a change in the optical characteristics of the projection optical system, which results from a change in illumination conditions, in the exposure apparatus has been strengthened. The necessity of measuring a change in the optical characteristics caused by the influence of heat generated during exposure of the wafer has also been strengthened.
Japanese Patent Laid-Open Nos. 2000-277412 and 2003-254725 disclose interferometers each of which is installed in an exposure apparatus, allowing the optical characteristics of a projection optical system to be measured in the exposure apparatus. The interferometers can be, for example, a point diffraction interferometer or a shearing interferometer.
In the discussed interferometers, a member having a fine pattern, such as a pinhole, a slit, or a diffraction grating, needs to be placed on each of a reticle side and a wafer side of the projection optical system.
For example, pinholes are sometimes necessary on the reticle side and the wafer side of the projection optical system in a point diffraction interferometer, and a diffraction grating can be necessary on one of the sides. A pinhole or a diffraction grating can be necessary on the reticle side and a diffraction grating can be necessary on the wafer side in a shearing interferometer.
However, in an immersion exposure apparatus, the distance between a projection optical system and a wafer is quite short because immersion liquid is held therebetween. For this reason, it is quite difficult to place a member having a fine pattern and a driving mechanism for driving the member between the projection optical system and the wafer.
In order to generate an ideal wavefront or separate the wavefront by using a fine pattern, machining accuracy for the fine pattern is important. As the numerical aperture of the projection optical system increases, the required density of the pattern increases, machining becomes more difficult, and machining errors arise more easily. When machining errors arise, an obtained wavefront deviates from an ideal wavefront, or the wavefronts are not precisely separated. This may decrease the accuracy in measuring the performance of the projection optical system. In addition to the machining errors, the amount of light passing through the fine pattern is reduced by increasing the density of the pattern. This may also decrease the measuring accuracy.
When a diffraction grating is provided on the wafer side, ±1-order light from the diffraction grating becomes an evanescent wave, that is, no light beam sometimes comes from the diffraction grating.
Although slightly dependent on the pitch of the diffraction grating and the position of the sensor, as the numerical aperture of the projection optical system increases, the shear ratio (the amount of shear between two separated wavefronts) greatly varies depending on the position. Therefore, it is difficult to precisely measure the performance of the projection optical system over the entire pupil plane of the projection optical system.
The reduction ratio of the projection optical system in the exposure apparatus is typically ¼ to ⅙. Therefore, the numerical aperture on the reticle side is lower than on the wafer side. For this reason, even when a fine pattern is placed on the reticle side, it has a size larger than on the wafer side, and an extremely high machining accuracy is not required in most cases. Accordingly, the fine pattern can be placed on the reticle side without any serious problems.